Light-emitting diodes (LEDs) can be used in many applications such as general lighting. LEDs offer several advantages over traditional light sources such as fluorescent lamps and incandescent lamps. For example, LEDs have significantly lower power consumption. Unlike traditional light sources such as incandescent light bulbs that convert a significant amount of electrical current heating up metal filaments to a temperature high enough to generate light, LEDs generate virtually no heat and utilize a fraction of the energy to produce an equivalent lumen of lighting. For example, in a light bulb, an LED light source may consume less than seven Watts to produce the same amount of brightness as an incandescent light source consuming approximately 60 Watts.
Furthermore, the operational life of an LED can be extended to over 50,000 hours, which is significantly longer than the average life of an incandescent bulb, e.g., 5000 hours, and the average life of a fluorescent lamp, e.g., 15,000 hours. Moreover, LEDs contain no mercury or any other hazardous materials or chemicals and emit zero ultraviolet (UV) radiation unlike incandescent or fluorescent lamps. The use of the LEDs materially enhances the environment and conserves energy.
Conventionally, an AC/DC converter converts an AC (alternating current) voltage to a substantial DC (direct current) voltage to power the LEDs. FIG. 1 illustrates a conventional driving circuit 100 for driving a light source 120, e.g., an LED array. The driving circuit 100 includes a rectifier 104, a capacitor 106, a control unit 108 (e.g., an operational amplifier), a switch 110 coupled in series with the light source 120, and a current sensor 114 (e.g., a resistor). The rectifier 104 can be a bridge rectifier including four diodes for rectifying an AC voltage VAC from a power source 102 to a rectified AC voltage. The capacitor 106 filters the rectified AC voltage and provides a substantially constant DC voltage VDC. The capacitor 106 can be an electrolytic capacitor which is relatively large in size. The current sensor 114 senses a current through the light source 120 and provides a sensing signal 116 to the control unit 108. The control unit 108 linearly controls the switch 110 based on a preset current reference 112 and the sensing signal 116 to control the current through the light source 120.
The drawback of the conventional driving circuit 100 is that the power efficiency of the driving circuit 100 decreases (e.g., from 84% to 71%) if the input AC voltage increases (e.g., from 210V to 250V). In other words, a greater input AC voltage can result in larger power loss and poor heat dissipation.